1. Field of the Invention
The present invention relates generally to the manufacture of integrated circuits and, in particular, to detecting and monitoring transmission degradation of reticles used in lithographic production systems.
2. Description of Related Art
In the semiconductor industry, photolithography systems are used to transfer patterns or shapes from a reticle (i.e., photomask) to a semiconductor wafer. These patterns may be transferred a number of times across the wafer to form microelectronic circuits or other semiconductor device features.
In a typical photolithographic process, a thin layer of a photosensitive material (e.g., photo resist) is deposited over a semiconductor wafer, which may have many chips on a surface thereof. The reticle is made from a transparent plate often comprising glass, quartz, or the like. The reticle includes a device exposure region and a patterned opaque region typically comprising a chrome layer or region. The opaque pattern on the reticle represents a desired layout of a circuit or semiconductor feature.
During the photolithography process, the patterned reticle is illuminated with radiation (e.g., laser radiation or radiation from an arc lamp) in the exposure region. The reticle exposure region generally has a square or rectangular shape and is positioned in the center of the reticle. It includes both the transparent portions and opaque portions that together define the device pattern. In illuminating this exposure region, a portion of the radiation is allowed to pass through the transparent portions, while the opaque portions block another portion of the radiation. A projection lens having an image field of a given size collects the radiation that is passed through the transparent portions, and projects the reticle pattern onto the photo resist layer. This pattern creates exposed and unexposed regions on the chip(s) residing on the wafer surface, whereby the exposed regions are then removed to define the desired circuit layout on the wafer. The photolithographic process of transferring images from a patterned reticle onto a wafer may be repeated many times across the same chip on the wafer, on different chips on the wafer, or even on several chips across different lots of wafers.
In transferring the image patterns, a single patterned reticle may be repeatedly used to transfer numerous images onto the chip(s) residing across the wafer(s). However, over time and use of the reticle degradation gradually occurs to the reticle's device exposure regions. Conventional approaches aimed at solving this problem include monitoring the patterns transferred on the wafers. In these approaches any unacceptable deviations amongst the patterns transferred onto the wafer indicate that the reticle is no longer suitable for use in lithography. However, these methods are time consuming since many locations must be monitored across the wafer for detecting any such deviations. They are also costly since the deviations are not detected until the images are transferred onto the wafers, thereby leading to increased wafer scrap and decreased production yields.
Other approaches aimed at determining whether acceptable images are being transferred onto a wafer include the use of test reticles to monitor deviations, the use of metrology tools to measure and control the characteristics of the patterns formed on the wafer by adjusting the lithographic processing conditions, as well as manually inspecting both the reticle patterns and the transferred images to ensure that the transferred images are within acceptable parameters of the desired layout specifications. However, all of these approaches are time consuming, inefficient, expensive and increase scrap materials, which in turn, lead to decreased production yields.
Accordingly, a need continues to exist in the art for easily and efficiently detecting and monitoring transmission degradation of reticles used in photolithography production systems.